Exponential deflecting and centering circuits



M. H. MESNER y29412963 EXPONENTIAL DEFLECI'ING AND CENTERING CIRCUITS De@ 17, was.

Filed Aug. 20, 1942 mventor ttorneg Patented Dec. 17, 1946 Unire. sra'rs EXPONIENTIAL DEFLECTING AND CENTERING CIRCUITS Radio Corporation o of Delaware Application August 20, 1942, Serial No. 455,409

My invention relates to cathode ray deiiecting circuits and particularly to centering means for deflecting circuits of the exponential deflection type used, for example, in radio pulse-echo systems for measuring the angle formed by a line from an aircraft to a ship or other target and a perpendicular line fromthe aircraft to earth. The present application claims the improved centering circuit which is described but not claimed in application Serial No. 453,088, filed July xS1, 1942, in the name of William D. Hershberger.

An object of the invention is to provide an improved centering circuit for cathode ray deection circuits.

A further object of the invention is to provide -in a cathode ray push-pull deilecting circuit an improved means for adjusting one end of the deflection sweep.

A still further object of the invention is to provide in apparatus of the above-described type an improved centering circuit which is so designed that one end of the deection sweep will not change its position even though the amplitude of the sweep is changed.

The invention will be described as applied to a radio pulse-echo system which radiates a signal downward and forward from an aircraft whereby reiiections that are a measure of altitude will be received from the earth directly below the plane while other reflections will be received from the target ahead.

Since the iirst occurring reection will be a` measure of the distance A perpendicular to the earth, the angle between this perpendicular line and the line from the aircraft to the target can be determined when the distance T1 to the target is also known. The latter distance T1 is shown by a later occurring reflection from the target. From the fact that the distance to the target divided by the altitude is the secant of the angle s, the angle 0 may be calculated. As described in the above-mentioned Hershberger application however, the apparatus is so `.designed that the angle 0 may be read directly off a scale without the necessity of any calculations.

Preferably my invention is applied to a specially designed push-pull circuit which generates an exponential sweep or timing voltage that is ap plied to one pair of deilecting plates of a cathode ray tube, the reflected altitude 'and target pulses representing distances A and T1, respectively,' being applied to a second pair of deiiecting plates in the tube.

In practicing a preferred embodiment of the 4 Claims. (Cl. Z50-27) fleeting plates 2l' and 21a.

l ward and lforward from a Ainvention I take advantage of the fact that the capacitors of the sawtooth circuit are completely discharged at the en 'Ihe power supply d of each exponential sweep. units for supplying charging voltages to the capacitors are held above ground potential and one terminal cf each power supply unit is direct-current connected to a point of proper potential in the centering circuit. Thus, at the time of complete discharge of the capacitors the only potential applied plates is the centering potential. As a result, the position of one end of the exponential sweep can readily be centered since its position is determined solely by the centering potentials and is independent of the power supply potential.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which the single gure is a block and circuit diagram of apparatus embodying my invention.

Referring to the single feature of the drawing,

-a high frequency radio transmitter l2 is keyed by means of a pulse keyer i3 and a keying pulse generator i4 to radiate short radio pulses downsuitable directive antenna system I6. The transmitter pulses are also applied over a conductor 20 to a receiver 22 to function as initial or time reference pulses. The keying pulses preferably are applied to the keyer I3 through a delay circuit I5. The antenna system I6 may be two dipoles in parabolic reflectors which radiate in overlapping left-hand and right-hand radiation patterns, only one dipole and reiiector being shown in the drawing. The two dipoles are connected alternately to the transmitter l2 by means of suitable left-right switching means Il driven by a motor I8. The transmitted pulses may occur at the rate of 3500 per second and have a duration of 0.3 microsecond; the switching from left to rightv may occur at the rate of per second. These values are given merely by way of example.

- The reflected pulses are picked up by a suitable antenna such as a dipole 2l and supplied to the receiver 22 which amplifies and demodulates them. They are further amplied by an amplifier 23 and applied through switch arms 24 and 25 to a pair of dei'lecting plates 2l and 21a of a cathode raytube 29.v A resistor 26 with a. grounded midpoint is'connected across the de-l The switch arms 24 and 25 are operated synchronously with the leftright antenna switching whereby the cathode ray of tube 29 is deflected to give a left indication Awhen the antenna radiation is toward the left to the deflecting age source d2.

and to give a right indication when the antenna radiation is toward the right. rlfhe alternate closing of switches 2d and is elected by means of a suitable left-right switching mechanism driven by the motor i8.

The cathode ray tube 29 may be of conventional design and is provided with a second pair of deflecting plates 3i and Bla positioned at right angles to the other pair of plates. The eX- ponential sweeper timing voltage is applied to the plates E! and Sla to deiiect the cathode ray vertically at a repetition rate of 3500 sweeps per second in the example being described. rlhis voltage is produced by the push-pull deflecting circuit which comprises a variable capacitor 35 associated with a cathode loaded tube 37. The tube S4 functions as a discharge tube for capacitor 33 and may be a screen grid tube which is biased beyond plate current cutoi by means of grid leak biasing provided by a resistor 39 and a capacitor il in the control grid circuit, for example. The grid circuit of tube Sil preferably includes a low resistance resistor 5 for reducing parasitic oscillations. Capacitor 33 receives a charge from a Voltage source 42 through the tube til upon the application of a pulse P to the control grid i8 of tube 34 from the pulse generator lli. The output circuit of source 42 includes the usual bleeder resistor 8| and iilter capacitor 82. The pulses P are applied trom a generator Ul through a conductor td, an amplifier tube A6, an amplier tube il and a transformer 4B which has a secondary winding 6 that is damped by a shunting resistor.

The charging path traced from the positive source d2 through a conductor for capacitor 33 may be terminal of voltage 3E and a section of the voltage divider 5S and `its bypass capaci tor 3S to ground, through ground to the lower terminal of capacitor 33, and through the capacitor S3 and the plate-cathode impedance of the tube 3ft to the negative terminal of the volt- At the end of the pulse P the capacitor 35 discharges through a path that may be traced from the upper terminal of .capacitor 33 through the plate resistor i3 of tube 3B, the conductorV 35, the section of voltage divider 58 and its bypass capacitor 38 to ground,

and through ground to the lower terminal oy capacitor 33. Y v

It 'should be noted that the capacitor 33 always discharges completely to bring its upper terminal (and also the deecting plate 3|) to the centering potential at the centering tap Il. It is at the end of the discharge period that the cathode ray is at the upper end of its sweep. It follows that changing the amplitude of voltage from source i2 does not alect the centering at this end of the sweep so far as capacitor 33 is concerned. The same is true with respect to capacitor 35 as will immediately be apparent. The lower terminal of capacitor 33 may be connected to the negative terminal of voltage source 42 instead of to ground, if desired.

The sweep voltage appearing across capacitor E3 is shown at 5l. It has a wave shape that may be described as a non-linear sawtooth wave shape, the useful deilecting portion of the sawtooth' beingv exponential, that is, being bent-- over in accordance with the discharging of capacitor 33 through resisto-r 43 at an exponential rate. Preferably during each pulse P the capacitor 33 is fully charged and at the endof each discharging period it is completely discharged.

. of the wave 53 is obtained by connecting the capacitor v24S in the cathode circuit of tube 3'? rather than in the plate circuit. Capacitor 36 receives. its charge from a voltage supply 5t kthrough the anode cath-ode impedance of tube 3l when said impedance is lowered the occurrence of a voltage pulse l? as will be explained below. The output circuit of source dii includes the usual bleederresistor 33 and ilter capacitor 84. VThe tube Cil, like the tube Sii, is normally biased to cut-off by ineans of grid circuit elements Q5 and 5E and preferably is provided with a parasitic reducing resistor i. The path of the charging current may be traced from the positive terminal of source et through the tube 3l, the capacitor to ground, from ground through a portion of voltage divider resistor 5B to a centering tap lil, and through a conductor 5S to the other terminal of the source At the end of a pulse P the capacitor 3G discharges through a cathode resisto-r Si, the conductor di? and the portion of the voltage divider 53 located between the centering tap l) and gro-und. Here again the capacitor discharges completely bringing the upper terminal of capacitor 3G to the 'centering potential at tap l@ whereby the upper end position of the cathode ray sweep is independent of changes in the voltage amplitude of source 55.

In 'the particular equipment being described, the time constant of the sweep circuit iis-d3 (or circuit Sii-5l) when set to measure the maximum altitude such as 20,000 reet is about onefth the period of the Vertical sweep, this being the condition for maximum resistance and capacity in the circuit and for the least amount of discharge of the capacitor 33 (and of capacitor 36) at the end of the sweep. Even for this condition capacitors 3S and 36 are substantially completely discharged; specifically, if the charging voltage from source iii is 2G00 volts, the capacitor 33 discharges tc 7 or 8 volts which is such a small percentage of the charging voltage that ther failure to discharge to zero volts does not affect the operation.

Referring again to the charging period for capacitor 36, the plate-cathode impedance oi tube 37 is lowered during the occurrence of each pulse P by applying these pulses simultaneously to its control grid 62 and to its screen grid 63. The control grid G2 receives its pulse through the secondary 6 of transformer d8 while the screen grid 63 receives its pulse through an'amplier tube 64 and a transformer (it, the tube 54 being Acoupled to the anode circuit oi ampliiier tube 45.

impedance by a pulse P of reasonable amplitude'.

The reason for applying the voltage pulses to the screen grid 63 as well as to the control .grid 62 is that the cathode 6l swings vpositive above ground potential when capacitor 36 receives` its transformers 48 and 66 lhave low capacity with respect to the primary windings whereby the secondary windings can readily swing above ground potential.

The sweep circuit does not defocus the cathode ray during the sweep deflection because the deecting voltage waves 5l and 53 swing the deecting plates 3| and 35a, respectively, about ground potential, this also being substantially the potential of the second anode 68 of cathode ray tube 29. The second anode potential may be adjusted by means of a movable tap 69 on a potentiometer resistor 1I which has its ends connected vto opposite sides of a grounded point on the voltage divider 58.

The voltage sources 42 and 5B are rectierfilter units supplied with power from an A.C. line through a voltage control unit 'l2 such as a Variac and through a transformer 13. Adjustment-of the tap marked Vertical size adjusts the amplitude of the vertical sweep. The primary winding of transformer 13 has a midtap and an end tap, indicated by the legends Angle and Let-up, respectively. By moving a switch arm 'I4 from its Let-up position to its Angle position the voltage supplied by the units 42 and 56 is doubled and the amplitude of the vertical sweep is doubled.

The variab-le capacitors 33 and 36 are mechanically coupled to a dial pointer 16 associated with an altitude scale 17. Thus the pointer 16 rotates when a knob 'I8 is rotated to change the capacity of capacitors 33 and 36 for the purpose of Set up and for obtaining altitude as explained below. In the example being described each of the capacitors 33 and 36 has a capacity range from 150 mmf. to 550 mmf.

The procedure in setting up and reading the altitude and angle scales, is as follows:

(1) The switch '14 is thrown to the Set up position shown in the drawing so that one-half full deflection voltage is applied from capacitors 33 and 36 to the vertical deflecting plates 3| and 31a.

(2) The end of the vertical sweep or cathode I ray trace is brought opposite the 90 degree mark at the top of the angle scale by adjusting the centering tap l0 on voltage divider 58.

(3) The initial or time reference pulse is then brought opposite the zero mark at the bottom of the angle scale by means of the Vertical size tap. With my improved centering circuit it is not necessary to re-center to keep the end of the trace opposite the 90 degree mark since the capacitors 33 and 36 always discharge to the centering potentials whereby the upper end of the trace remains xed.

(4) The altitude echo pulse indication is next;

tion ofthe knob 18 also rotates the pointer 'i6` of the. altitude scale from which the altitude "i may now be read.

(5) The switch 'I4 is next moved to its Angle measuring position putting the full sweep voltage on the vertical deflecting plates whereby lil 6 the length of the vertical sweep is doubled. This brings the altitude pulse indication to the zero mark on the angle scale. As noted above, the upper end of the sweeptrace does not change its position and no further centering is required,

(6) The angle 0 for a target that reflects a pulse T1 maynow be read directly from the point on the angle scale opposite the pulse T1 indication. A

The reason that the angle 6 and the altitude can be determined in the' manner described abovehas been fully described in the above-identified Hershberger application.

In the drawing the values of various circuit elements have vbeen indicated, merely by way of example, in ohms andmicrofarads.

' I claim as my invention: y

1. A cathode ray deilecting circuit comprising an electric discharge tubehaving a control grid circuit and an anodecircuit, a capacitor located in said anode circuit, means for alternately substantially completely charging said capacitor and substantially completely discharging said capacitor in response to the application of periodically recurring pulses to said control grid circuit, a

second electric discharge tube having a control grid, an anode and a cathode, means for so applying said pulses to lsaid last named grid as to drive it in the positive direction with respect to the cathode of said second tube, and a capacitor connected between said cathode and a point that is negative with respect to said cathode, means for alternately substantially completely charging said last capacitor and substantially completely discharging said last capacitor in response to the application of said pulses to the said grid 0f said second tube, and centering circuit means for bringing the anode end of the first capacitor and the cathode end of the second capacitor to predetermined direct-current potentials, respectively, at the end of each capacitor discharge.

2. In a deflecting circuit, an anode loaded electric discharge tube having a control grid, an anode and a cathode, a direct current power supply having positive and negative terminals, said positive terminal being connected to said anode through a plate resistor, said negative terminal being connected to said cathode, a capacitor having one end connected to said anode and the other end connected through a low impedance alternating-current path to said cathode whereby a sawtooth voltage is produced across said capacitor in response to the application of positive voltage pulses to said control grid, said other end of the capacitor also being connected through a low impedance alternating-current path to a common point in the deecting circuit, a centering circuit having positive and negative terminals and having an intermediate voltage point connected to a point about which the deecting voltage is to swing symmetrically, a direct current connection between the positive terminals of said power supply and said centering circuit whereby the anode end of said capacitor goes to said positive centering potential when said capacitor is discharged, a cathode loaded discharge tube having a control grid, an anode and a cathode, a second direct current power supply having positive and negative terminals, said positive teranode of said last tube, the negative terminal of said' second power supply being connected to said last-mentioned cathode through a resistor whereby a sawtooth voltage appears across said lastmentioned capacitor in response to the application of positive Voltage pulses to the control grid of said cathode loaded tube,- and a direct current connection between the negative terminals of said second power supply and said centering'circuit whereby the cathode end o1" saidYlast-mentioned capacitor goes to said negativecentering potential when said last-mentioned capacitor is discharged.

3. In combination, an anode loaded electric discharge tube having a control grid, an anode and a cathode, a direct -current power supply having positive and negative terminals, said positive terminal being connected to said anode through a plate resistor, said negative terminal being connected to said cathode, a capacitor having one end connected to said anode and the other end connected through a low impedance alternatingcurrent path to said cathode whereby a sawtooth voltage is produced across saidcapacitor in response to the application of positive voltage pulses to said control grid, a centering circuit having a positive terminal and having a point that is negative with respect to said terminal, said other end of the capacitor also being connected through a low impedance alternating-current path to said negative point of the centering circuit, and a diloaded discharge and negative terminals, said positive terminal being connected directly to the anode of said tube, a capacitor having one end connected to the cathodel of said tube and the other end connected through a` low impedance alternatingcurrent path to the anode of said-tube, the negative terminal of said power supply being con nected to said cathode through a resistor whereby a sawtooth voltage appears across said capacitor in response to the application of positive voltage pulses to said controlgrid,'a centering circuit having a negative terminal and having a point that is positive with respect to said terminal, said positive point being connected through a low impedance alternating-current path' to said other end of the capacitor, and a direct current connection between the negative terminals of said power supply and said centering circuit whereby the cathode end of said capacitor goes to said negative centering potential when said capacitor is discharged.

' MAX H. MESNER.

grid, an anode and a cath-y ode, a direct current power supply having positivel 

